1. Field of the Invention
This invention relates generally to phase-change memory cells. More specifically, the present invention relates to a phase-change memory cell for storing information in a plurality of programmable cell states.
2. Description of Related Art
Phase-change memory (PCM) is a non-volatile solid-state memory technology that exploits the reversible, thermally-assisted switching of phase-change materials, in particular chalcogenide compounds such as GST (Germanium-Antimony-Tellurium), between states with different electrical resistance. The fundamental storage unit (the “cell”) can be programmed into a number of different states, or levels, which exhibit different resistance characteristics. The s programmable cell-states can be used to represent different data values, permitting storage of information.
In single-level PCM devices, each cell can be set to one of s=2 states, a “SET” state and a “RESET” state, permitting storage of one bit per cell. In the RESET state, which corresponds to a wholly amorphous state of the phase-change material, the electrical resistance of the cell is very high. By heating to a temperature above its crystallization point and then cooling, the phase-change material can be transformed into a low-resistance, fully-crystalline state. This low-resistance state provides the SET state of the cell. If the cell is then heated to a high temperature, above the melting point of the phase-change material, the material reverts to the fully-amorphous RESET state on rapid cooling. In multilevel PCM devices, the cell can be set to s>2 programmable states permitting storage of more than one bit per cell. The different programmable states correspond to different relative proportions of the amorphous and crystalline phases within the volume of phase-change material. In particular, in addition to the two states used for single-level operation, multilevel cells exploit intermediate states in which the cell contains different volumes of the amorphous phase within the otherwise crystalline PCM material. Since the two material phases exhibit a large resistance contrast, varying the size of the amorphous phase within the overall cell volume produces a corresponding variation in cell resistance.
Reading and writing of data in PCM cells is achieved by applying appropriate voltages to the phase-change material via a pair of electrodes associated with each cell. In a write operation, the resulting programming signal causes Joule heating of the phase-change material to an appropriate temperature to induce the desired cell-state on cooling. Reading of PCM cells is performed using cell resistance as a metric for cell-state. An applied read voltage causes a current to flow through the cell, this read current being dependent on resistance of the cell. Measurement of the cell read current therefore provides an indication of the programmed cell state. A sufficiently low read voltage is used for this resistance metric to ensure that application of the read voltage does not disturb the programmed cell state. Cell state detection can then be performed by comparing the resistance metric with predefined reference levels for the s programmable cell-states.
The development of multilevel PCM faces several key challenges. One of these is that the amorphous phase of phase-change materials exhibits undesirable attributes such as low-frequency noise and drift. This drift causes resistance of the amorphous phase to increase in value over time. As a result, the read measurements for programmed cell states tend to vary with time. This complicates read out of the stored information, potentially even destroying the information if there is a large variability in the drift exhibited by different cell states so that the read measurement distributions for neighboring cell states interfere with one another. The larger the number of cell states, and so closer the initial spacing between readback resistance levels, the more susceptible cells are to this problem.
Various techniques have been proposed to alleviate problems associated with resistance drift. One approach is disclosed in European Patent Application publication no. EP 2034536 A1 and illustrated in FIG. 1 of the accompanying drawings. This figure shows a schematic illustration of a PCM cell 1 having a volume of phase-change material 2 located between a top electrode 3 and a bottom electrode (or “heater”) 4. The cell state shown represents an intermediate state in which the material 2 contains both crystalline and amorphous phases. The amorphous phase is indicated by the shaded hemispherical volume 5 above bottom electrode 4. The crystalline phase 6 occupies the remainder of the cell volume. A thin resistive region 7 provides a parallel current path between the bottom electrode 4 and the crystalline phase 6 of the phase-change material in operation. When a read voltage is applied to read the programmed cell-state, the resulting read current flows primarily via this current path from crystalline phase 6 to bottom electrode 4, in preference to flowing through the high-resistance amorphous phase 5. The measured cell resistance thus depends primarily on resistance of the parallel current path rather than the resistance of amorphous phase 5. The resistance of the parallel current path depends on the length “x” in the figure. This length is dependent on the size of amorphous phase 5, and thus varies with programmed cell state. Since the resistance of element 7 is unaffected by drift, the effect on the read measurement of resistance drift in amorphous phase 5 is mitigated.
Cell efficiency, which depends on the current/power required to program a PCM cell, presents another challenge. The cell efficiency can be improved by reducing the cell dimensions, but this increases the overall resistance of the cell. This makes it difficult to sense the high resistance levels associated with the multiple programmed states.
A further challenge is the occurrence of “thermal disturb”. This refers to thermal cross-talk between a cell being programmed and its neighboring cell which may disturb the programmed state of the neighboring cell.
Current PCM cell designs fall into two main categories: mushroom cells, also called planar cells, and confined cells. Mushroom cells, of which variants include the micro-trench cell, have the general form of the FIG. 1 cell, with a volume of PCM material having a small contact area with the underlying bottom electrode. Confined cells, of which variants include the keyhole or pore cell, have the general form shown in FIG. 2 of the accompanying drawings. Here, a smaller volume of PCM material is confined between the top and bottom electrodes. Mushroom cells are easier to fabricate than confined cells, but have serious issues with cell efficiency and thermal disturb in addition to the drift effects discussed above. Confined cells offer improved cell efficiency, but fabrication of these confined cell structures is especially challenging, particularly if filling of high-aspect ratio trenches with chalcogenides is required. One example of a fabrication method for a confined cell is described in United States Patent Application Publication US 2010/0163817A1.
Further improvements in PCM cells are desirable.